1. Field of the Invention
This invention is concerned with a continuous or semi-continuous method for producing highly oriented graphite crystals.
2. Description of the Prior Art
The element carbon exists in three allotropic forms which are employed industrially--amorphous carbon, graphite and diamond. Carbon is also a major constituent of coal. In general, carbon is inert and infusible at atmospheric pressure. Some of the industrial applications of carbon depend upon its chemical inertness. One of the most useful forms of carbon is graphite. Graphite is a soft crystalline modification of carbon that differs greatly in its properties from both amorphous carbon and diamond.
Graphite, as it occurs naturally, has been known to man for many centuries. By the Middle Ages it was being employed for writing and drawing purposes. Natural graphite was the only kind available, except in laboratory quantities, until the first successful commercial process for artificial graphite was introduced by Edward Acheson in 1896. Acheson placed amorphous carbon together with certain catalysts such as silica or alumina in a furnace at temperatures about 3000.degree. C. to produce graphite.
Graphite has unique properties which make it a very valuable material. Graphite has low friction and wear properties which render graphite useful as a lubricant and in bearings and seals. Since graphite is fairly chemically inert, it can be used with corrosive fluids, e.g., acids, salt solutions, alkalies and organic compounds. The easy-machining qualities of graphite, together with its availability in many sizes and shapes, permit construction of a complete range of process equipment, including heat exchangers, pumps, valves, pipes, fittings, towers, tank linings, storage chambers and absorbers. Foundry facings constitute the largest single use of natural graphite, followed by lubricants, steelmaking, refractories and crucibles.
Even though graphite is a non-metal, graphite possesses very good electrical and thermal properties. The electrical industry uses graphite for electrodes, brushes, contacts, and electronic-tube rectifier elements. The high thermal conductivity of graphite is particularly important in heat-exchange applications. Graphite is used in the construction of shell and tube plate-type and double-pipe heat exchangers. Graphite is also highly resistant to thermal shock.
In the field of metallurgy, graphite is employed for electrodes in carbon-arc welding and in electric arc furnaces for steel melting. In other fields of metallurgical technology, graphite is utilized for hot-pressing dies, as susceptors in induction furnances, molds, and melting crucibles.
Structural shapes of extremely high purity graphite have become of great importance in nuclear energy applications, particularly nuclear reactors. Graphite blocks are used to hold the slugs of nuclear fuel in position in the space lattice which forms the nuclear reactor. The primary function of graphite, however, is to act as a moderator for the reactor core. Graphite is also used as a coating for uranium fuel and as a container for nuclear matter.
Graphite is now being extensively used in the field of space technology. Rocket nozzles and nose cones for space vehicles are being fabricated from graphite.
Commercial users of graphite include the pencil industry and the paint industry. Graphite is also used as a molecular sieve for absorbing gases and as a catalyst support. Another use for graphite is as a surgical implant, since graphite can be used safely in contact with body fluids.
Graphite can be derived from natural sources. United States production of graphite has been practically nonexistant in recent years. Most of our supply of natural graphite is from imports, with Mexico being the prime supplier. Artificial graphite can be substituted for almost any of the uses of the natural product, and in many instances, high purity artificial graphite is preferred.
It is known that carbons of widely different properties can be prepared by starting with different carbonaceous materials and treating these materials in various ways. Carbons can be produced by starting with organic precursors, such as oil, coal, natural gas, organic compounds and polymers and heating such precursors at a sufficiently high temperature to liberate hydrogen, oxygen, nitrogen and sulfur.
Artificial graphite can be made electrically from retort or petroleum coke. Coke or vitreous carbon is heated to temperatures in excess of 2200.degree. C. under hydrostatic pressure, with or without catalysts. By this method, the highly turbostatic starting material is formed into well crystallized graphite. Another approach is to grow graphite crystals from solution. The growth of graphite single crystals from a carbon-saturated metallic melt is taught by Austerman, Myron and Wagner, Growth and Characterization of Graphite Single Crystals, CARBON, Vol. 5, pp. 549-557, 1967, and Noda, Sumiyoshi and Ito, Growth of Single Crystals of Graphite from a Carbon-Iron Melt, CARBON, Vol. 6, pp. 813816, 1968.
U.S. Pat. No. 3,664,813 describes a process for making graphite whiskers on a metal covered substrate, wherein said substrate is in a whisker deposition zone of hydrocarbon whisker vapor and where there is a temperature gradient around said substrate. The growing whiskers are thus deposited on the substrate.
In U.S. Pat. No. 4,048,953, an apparatus is disclosed for the vapor deposition of pyrolytic carbon on porous sheets of carbon material. The pyrolytic carbon is deposited from a hydrocarbon gas which is heated to a very high temperature, i.e., 2000.degree.-2400.degree. C.
The formation of graphite films by dissolving carbon in cobalt and nickel at high temperatures with the film being produced by precipitation on cooling is discussed in the following references: F. J. Derbyshire, A. F. B. Presland and D. L. Trimm, CARBON, Vol. 10, pp. 114-115, (1972); F. J. Derbyshire, A. F. B. Presland and D. L. Trimm, Graphite Formation By The Dissolution-Precipitation of Carbon in Cobalt, Nickel and Iron, CARBON, Vol. 13, pp. 111-113, 1975; F. J. Derbyshire and D. L. Trimm, Kinetics of the Deposition of Pyrolytic Carbon on Nickel, CARBON, Vol. 13 pp. 189-192, 1975; R. T. K. Baker, P. S. Harris, J. Henderson, and R. B. Thomas, Formation of Carbonaceous Deposits from the Reaction of Methane Over Nickel, CARBON, Vol. 13, pp. 17-22, 1975.
Thus, the prior art discloses the use of heated Group VIII metals to deposit graphite via diffusion and subsequent precipitation on a batch-type basis. In prior art methods, after diffusion of carbon occurs through the heated metal, the carbon is deposited on a surface of the metal by allowing the metal to cool down and by thereafter collecting the precipitated graphite in a batch-type operation.
It is an object of the present invention to provide a process for the production of highly oriented graphite crystals in a continuous or semi-continuous operation. This and other objectives can be realized by means of the invention described herein.